1. Field of the Invention
The present invention relates to a sealed cell, and more specifically, to the improvement of a sealing body with a safety valve in the sealed cell.
2. Background Art
Non-aqueous electrolyte secondary cells are widely used as the driving power sources of portable devices and electric tools because of their high energy density and high capacity.
These cells using flammable organic solvents are required to ensure safety, and for this reason, the sealing body for sealing such a cell includes a current breaking mechanism and a gas releasing mechanism, which operate when the cell internal pressure increases.
FIG. 11 shows a sealed cell having a sealing body 10 including conventional current breaking and gas releasing mechanisms. As shown in FIG. 11, the sealing body 10 includes a terminal cap 5, a safety valve 3 disposed on the inner surface of the terminal cap 5, a terminal plate 1 disposed on the inner surface of the safety valve 3, and an insulating member 2 providing isolation and insulation between the safety valve 3 and the terminal plate 1. A current collector tab 8 connected to one of electrodes of an electrode assembly 40 is mounted to the terminal plate 1. In order to keep the conductive contact between a cap terminal 5 and a safety valve 3, the terminal cap 5 and the safety valve 3 are fixed to each other at the periphery. In addition, a convex is formed toward the inside of the cells near the center of the safety valve 3, and a portion near the top of the convex (a conductive contact portion 3a) is welded to the terminal plate 1. A break groove, a thin portion of the terminal plate 1, is formed on the terminal plate 1 at the periphery of the portion welded to the conductive contact portion 3a. 
The current breaking mechanism and the gas releasing mechanism of the sealed cell operates as follows. When the cell internal pressure increases, the convex of the safety valve 3 toward the inside of the cells (conductive contact portion 3a) is pushed up toward the outside of the cell. This results in the breakage of the break groove of the terminal plate 1 connected to the conductive contact portion 3a of the safety valve 3, thereby interrupting current supply to the terminal cap 5.
When the cell internal pressure further increases after the breakage of the terminal plate 1, a notch 3d of the safety valve 3 breaks. Thereby, an internal gas is released outside through the broken notch 3d and the gas releasing holes 5c. For this reason, the safety of the cell is further improved.
In the meanwhile, recently, further improvement in volume energy density of the cell is required. Therefore, it is sought to decrease a space of a projection of the terminal cap 5 because the space does not contribute to charge/discharge reaction. However, there is a problem that if the space is decreased, a gas releasing hole is apt to be blocked with a deformed safety valve, and thus sufficient safety is not available.
Well-known techniques on the sealing body of the sealed cell include the following Patent Documents 1 to 5.    Patent Document 1: Japanese Patent Unexamined Publication No. 2000-67837    Patent Document 2: Japanese Patent Unexamined Publication No. 2006-12604    Patent Document 3: Japanese Patent Unexamined Publication No. 2008-123726    Patent Document 4: Japanese Patent Unexamined Publication No. H06-203818    Patent Document 5: Japanese Utility Model Unexamined Publication No. S64-23873
Patent Document 1 describes a sealing plate including a groove, which is formed on the surface of the sealing plate, to cause a crack for releasing gas in the battery when the internal pressure of the battery reaches a predetermined value. And a predetermined area of the sealing plate is drawn into a bowl-like shape. According to this technique, even when a hard material such as stainless steal is used, deformation seldom occurs during processing, and furthermore a groove having a low working pressure enough for a practical use can be easily formed. However, there is a problem in this technique that an internal gas can be released but electrical current cannot be interrupted.
Patent Document 2 describes a technology using a group of sealing plates comprising a rapture plate having a fragile portion that can be easy to break due to increased gas pressure, the cyclic PTC element, a ring-shaped conductive support plate that comprises a thin film covering the space in the ring and having a thin film that can be easy to break due to increased gas pressure, and the terminal plate. In the group of sealing plates, the above-mentioned components are deposited in the above-mentioned order. According to this technology, the rapture plate works stably at the abnormal time such as overcharging, and thus the rupture of the cell can be prevented. However, in this technology, there is a problem of not being able to improve volume energy density of the cell.
Patent Document 3 describes a technology using a safety valve comprising an inner rupture groove and a radial groove that easily ruptures in a substantially radial direction. This radial groove extends toward the outer edge and substantially perpendicular to the inner rupture groove that is shaped in a continuous plane or a discontinuous loop. According to this technology, the above structure operates rapidly when the internal battery pressure is elevated, and can quickly exhaust internal gas. Moreover, this technology provides the safety valve having an excellent strength against falling. However, in this technology, although an internal gas can be released, there is a problem of not being able to interrupt electrical current.
Patent Document 4 describes a technology in which an explosion-proof valve that can deforms in the direction of the internal pressure depending on increased pressure in the battery is welded to a member for mounting a lead plate that is provided inside of the battery. This weld is detachable under a predetermined pressure. According to this technology, a pressure rise can be stopped at an early stage of the increase in the internal pressure, and a temperature rise due to charged current or short-circuit current can be prevented, and also firing or explosion of the battery can be effectively prevented. However, in this technology, there is a problem of not being able to improve volume energy density of batteries.
Patent Document 5 describes a technology to caulk and seal an opening of a outer battery can via a gasket with a first lid made of aluminum alloy or aluminum having 0.2-0.6 mm thickness and a second lid made of hard metal. The first lid comprises a substantially radial thin portion having 50-300 μm thickness and a flange on its periphery. Moreover, gas releasing holes are formed in the second lid. According to this technology, explosion-proof and strength of the battery can be secured. However, in this technology, although an internal gas can be released, there is a problem of not being able to block electrical current.